Multiple helicopter original equipment manufacturers (OEMs) are interested in hub-based active vibration control (AVC). Placing force generators on the helicopter hub provides the capability of canceling hub loads near the source, thus enabling global vibration control.
One type of hub-based force generator comprises two co-rotating motorized imbalanced rotors. These rotors may rotate in the same direction as the hub such that the masses are rotating at the blade pass frequency. These rotors create a controllable rotating force vector that can be controlled to cancel hub loads. Another type of hub-based force generator comprises two pairs of co-rotating motorized imbalanced rotors—one pair rotating in the same direction as the hub, and the other pair rotating in the opposite direction.
One challenge with hub-based AVC, however, pertains to the need to address certain failure modes such as loss of operation or loss of power in which the imbalanced rotors will discontinue rotating relative to the hub. They may come to a stop in a statically balanced condition, in a statically worst case imbalance condition, or in some condition in between. The static imbalance that results after loss of operation will create 1P hub loads that will cause vibration on the hub, the gearbox and the engines. The static imbalance condition that results after such a failure mode may dictate the severity of the failure mode (e.g., minor, major, hazardous, or catastrophic). For example, if after loss of operation, the rotors come to rest in a statically mass balanced condition, this may be classified as a Minor failure mode. On other hand, if after loss of operation, the rotors come to rest in a severely imbalanced condition, the resulting 1P loads and vibration may be severe enough that this may be classified as a Hazardous or catastrophic failure mode. If the latter is true, a hub-based AVC system design is required that will sufficiently mitigate this failure mode. Accordingly, there is a need for improvements to design and system architecture to address potentially hazardous and catastrophic failure modes.